Bovine Respiratory Syncytial Virus (BRSV)

Bovine Respiratory Syncytial Virus (BRSV) is a highly contagious viral pathogen that affects the respiratory system of cattle worldwide. This enveloped RNA virus belongs to the Pneumoviridae family and is closely related to human respiratory syncytial virus, making it a significant concern for both animal health and comparative medical research. BRSV primarily targets the epithelial cells lining the respiratory tract, causing inflammation, tissue damage, and compromised lung function that can range from mild respiratory distress to severe, life-threatening pneumonia.

BRSV affects cattle of all ages but demonstrates a particularly severe impact on young calves between two weeks and nine months of age. The virus is endemic in cattle populations across every continent, with seroprevalence studies indicating that the vast majority of adult cattle have been exposed to the virus at some point in their lives. Both dairy and beef operations experience BRSV outbreaks, though the stress associated with weaning, transportation, and commingling in feedlot settings significantly increases disease severity and transmission rates.

The economic impact of BRSV on the cattle industry is substantial and multifaceted. Direct losses include mortality, which can reach 20 percent or higher in naive herds experiencing their first outbreak, as well as the costs associated with treatment, veterinary care, and labor. Indirect economic consequences include reduced weight gain, decreased feed efficiency, delayed market readiness, and long-term respiratory compromise that affects lifetime productivity. In dairy operations, BRSV outbreaks can significantly impact heifer replacement programs when severe cases cause permanent lung damage in valuable young stock.

Early detection and prompt intervention are critical for managing BRSV infections effectively. While there is no specific antiviral treatment available for BRSV, supportive care combined with prevention of secondary bacterial infections can significantly improve outcomes. Vaccination programs, biosecurity protocols, and management strategies that reduce stress are essential components of a comprehensive BRSV control program. Working closely with a licensed veterinarian to develop herd-specific prevention and treatment protocols is essential for minimizing the impact of this economically significant disease.

Bovine Respiratory Syncytial Virus is caused by a single-stranded, negative-sense RNA virus belonging to the genus Orthopneumovirus within the family Pneumoviridae. The virus possesses a lipid envelope derived from the host cell membrane, making it relatively fragile in the environment but highly efficient at cell-to-cell transmission within the respiratory tract. BRSV replicates primarily in the ciliated epithelial cells of the respiratory tract, with the virus showing particular tropism for the bronchioles and alveoli of the lower respiratory system, where it causes the most significant pathological damage.

While BRSV itself does not have a genetic or hereditary component, certain cattle breeds and genetic lines may demonstrate varying degrees of susceptibility based on their immune response characteristics. Research has suggested that some cattle may possess genetic variants affecting interferon production and other immune parameters that influence disease severity. However, no specific breed is immune to BRSV infection, and all cattle populations maintain susceptibility to the virus regardless of their genetic background or breeding history.

Environmental and management factors play a crucial role in BRSV transmission and disease severity. The virus spreads primarily through respiratory secretions, including aerosol droplets produced during coughing, as well as through direct nose-to-nose contact between infected and susceptible animals. Crowded housing conditions, poor ventilation, dust accumulation, and high ammonia levels from inadequate waste management all contribute to increased transmission rates and more severe clinical disease. Seasonal patterns are evident, with BRSV outbreaks occurring more frequently during fall and winter months when cattle are housed indoors and environmental conditions favor viral survival.

Several risk factors significantly increase the likelihood and severity of BRSV infections in cattle populations. Young calves with waning maternal antibody protection are particularly vulnerable, as are cattle experiencing stress from weaning, transportation, dietary changes, or commingling with unfamiliar animals. Concurrent infections with other respiratory pathogens, including Mannheimia haemolytica, Pasteurella multocida, Histophilus somni, and bovine viral diarrhea virus, dramatically worsen clinical outcomes. Inadequate colostrum intake during the first hours of life leaves calves immunocompromised and more susceptible to severe BRSV disease.

The pathophysiology of BRSV infection involves complex interactions between viral replication and host immune responses. Following inhalation of virus particles, BRSV infects and destroys ciliated epithelial cells, compromising the mucociliary clearance mechanism that normally protects the lower respiratory tract from pathogens. The virus also infects alveolar macrophages and can trigger an exaggerated inflammatory response characterized by neutrophil infiltration, edema formation, and excessive mucus production. In severe cases, this inflammatory cascade leads to bronchiolitis, interstitial pneumonia, and emphysema, with some animals developing acute respiratory distress syndrome. The syncytia formation characteristic of this virus, where infected cells fuse to form multinucleated giant cells, contributes to tissue damage and impaired gas exchange.

Early warning signs of BRSV infection often appear subtly and may be easily overlooked in busy livestock operations. Initial symptoms typically include mild nasal discharge that may be clear or slightly cloudy, along with a soft cough that occurs sporadically throughout the day. Affected animals may demonstrate decreased appetite, appearing less enthusiastic at feeding time and spending more time standing away from feed bunks or hay racks. Body temperature may be mildly elevated, typically ranging from 103 to 104 degrees Fahrenheit in the early stages of infection. Careful observation may reveal slightly increased respiratory rate and reduced activity levels compared to herdmates.

As BRSV infection progresses, symptoms become more pronounced and easier to identify across different cattle species and ages. In young dairy calves, clinical signs often include obvious nasal discharge that progresses from serous to mucopurulent, excessive lacrimation with tear staining below the eyes, and frequent coughing episodes that may occur in paroxysms. Beef calves in feedlot settings commonly present with similar respiratory symptoms accompanied by depression, rough hair coat, and noticeable weight loss or failure to gain appropriately. Adult cattle, while typically experiencing milder clinical disease, may show decreased milk production in dairy cows or reduced feed efficiency in finishing cattle.

Behavioral changes associated with BRSV infection are significant indicators of disease presence and severity. Infected animals frequently separate themselves from the herd, standing alone in corners of pastures or pens rather than participating in normal social activities. Affected cattle often demonstrate reduced rumination, with decreased cud chewing observable during rest periods. Animals may exhibit reluctance to move, standing with heads lowered and extended necks to ease respiratory effort. In group housing situations, sick calves may be found lying down more frequently than healthy penmates and may resist standing when approached.

Physical examination of BRSV-infected cattle reveals characteristic signs of respiratory compromise. Auscultation of the lungs typically reveals increased bronchovesicular sounds, wheezes, and crackles, particularly in the cranioventral lung fields. Severely affected animals demonstrate open-mouth breathing, extended head and neck positioning, and visible abdominal effort during respiration. Subcutaneous emphysema may be palpable over the withers and back in animals with alveolar damage and air leakage into surrounding tissues. Rectal temperature frequently exceeds 104 degrees Fahrenheit during acute infection, though temperatures may normalize or become subnormal in moribund animals.

Symptom progression in BRSV follows a relatively predictable pattern when left untreated or in severe cases. The initial phase lasting two to three days features mild upper respiratory signs and low-grade fever. This progresses to an acute phase characterized by severe lower respiratory involvement, high fever, pronounced dyspnea, and significant depression. Some animals develop interstitial emphysema visible as crackling air pockets beneath the skin. Without intervention, severely affected cattle may progress to respiratory failure over the course of five to ten days. Secondary bacterial pneumonia frequently complicates the clinical picture, adding purulent nasal discharge, higher fevers, and consolidated lung sounds to the presentation.

Emergency symptoms requiring immediate veterinary intervention include severe respiratory distress with open-mouth breathing and foaming at the mouth, cyanosis of the mucous membranes indicating inadequate oxygen exchange, complete anorexia with inability to rise, rectal temperature exceeding 106 degrees Fahrenheit or falling below normal, and rapid deterioration of body condition. Calves that suddenly collapse or demonstrate extreme weakness need emergency care, as do animals showing signs of secondary bacterial septicemia including cold extremities, rapid weak pulse, and altered mentation. Any animal displaying subcutaneous emphysema extending beyond the thoracic region warrants urgent veterinary attention due to the risk of fatal respiratory failure.

Clinical examination forms the foundation of BRSV diagnosis in field settings. Veterinarians assess respiratory rate, effort, and character while auscultating the lungs for abnormal sounds including wheezes, crackles, and areas of consolidated or absent breath sounds. Physical examination includes evaluation of nasal discharge character, assessment of hydration status through skin tenting and mucous membrane moisture, and palpation for subcutaneous emphysema. Body temperature measurement, heart rate evaluation, and assessment of overall demeanor provide additional diagnostic information. The pattern of disease spread through a group, particularly affecting young calves in a sequential manner, strongly suggests BRSV involvement.

Diagnostic testing for BRSV employs several laboratory methods to confirm viral presence. Nasal swab or transtracheal wash samples can be submitted for virus isolation, polymerase chain reaction testing, or direct fluorescent antibody testing, with PCR providing the most sensitive and rapid results. Paired serology comparing acute and convalescent antibody titers can confirm recent infection, though this approach is more useful for retrospective herd diagnosis than for individual animal treatment decisions. In cases of mortality, necropsy examination with histopathology and immunohistochemistry of lung tissue provides definitive diagnosis. Fresh and fixed lung samples should be submitted to a veterinary diagnostic laboratory for comprehensive testing including bacterial culture to identify concurrent pathogens.

Differential diagnosis for BRSV includes numerous other causes of bovine respiratory disease. Infectious bovine rhinotracheitis caused by bovine herpesvirus-1 produces similar upper respiratory signs but typically affects the trachea more prominently. Bovine viral diarrhea virus can cause respiratory disease as part of a systemic infection and frequently occurs concurrently with BRSV. Parainfluenza-3 virus causes respiratory disease that is often clinically indistinguishable from BRSV without laboratory testing. Bacterial pathogens including Mannheimia haemolytica, Pasteurella multocida, Histophilus somni, and Mycoplasma bovis must be considered as primary or secondary causes of pneumonia. Non-infectious causes including aspiration pneumonia, toxic gas exposure, and allergic respiratory disease also warrant consideration.

Herd-level diagnostics provide valuable information for developing control strategies and understanding disease epidemiology within an operation. Bulk tank milk antibody testing in dairy herds can indicate recent BRSV circulation and help identify at-risk populations. Serological surveys of different age groups reveal patterns of exposure and help identify windows of susceptibility where vaccination might be most beneficial. Mortality data collection and analysis, combined with necropsy findings from representative cases, guides protocol development for treatment and prevention. Working with veterinary diagnostic laboratories to establish monitoring programs helps producers track BRSV activity and respond proactively to emerging outbreaks before extensive spread occurs.

Emergency and immediate treatment for severe BRSV cases focuses on supporting respiratory function and preventing death while the animal mounts an immune response. Severely dyspneic animals benefit from placement in well-ventilated areas with clean, dry bedding and minimal disturbance. Oxygen supplementation, when available, can be life-saving for animals in respiratory crisis. Anti-inflammatory medications, particularly non-steroidal anti-inflammatory drugs such as flunixin meglumine or meloxicam, help reduce fever, decrease inflammation in the airways, and improve patient comfort. Bronchodilator therapy may provide temporary relief for animals with severe bronchoconstriction, though response is variable.

Medical management of BRSV infection centers on supportive care and prevention of secondary bacterial complications. Antimicrobial therapy does not affect the virus itself but is essential for controlling the bacterial pneumonia that commonly follows viral respiratory damage. Appropriate antibiotic choices include tulathromycin, florfenicol, ceftiofur, or tilmicosin, selected based on veterinary guidance and knowledge of local resistance patterns. Producers must strictly observe withdrawal times for meat and milk when using any antimicrobial agents in food-producing animals. Anti-inflammatory therapy should continue for three to five days to reduce airway inflammation and support recovery. Fluid therapy may be indicated for dehydrated animals, administered either intravenously or subcutaneously depending on severity.

Surgical intervention is rarely indicated for uncomplicated BRSV infection. However, severe cases complicated by emphysema may occasionally benefit from needle decompression of subcutaneous air accumulation. Animals developing lung abscesses as a sequela to bacterial pneumonia following BRSV infection generally carry a poor prognosis and may require chronic antimicrobial therapy or culling decisions. Tracheostomy may be considered as a salvage procedure for animals with severe upper airway obstruction, though this is uncommon with BRSV unless concurrent conditions affect the upper respiratory tract.

Supportive care measures significantly impact treatment success and should not be overlooked. Providing high-quality, palatable feed encourages continued nutrition during illness. Fresh, clean water must be readily accessible, and electrolyte supplementation benefits dehydrated animals. Housing adjustments including improved ventilation, reduced stocking density, and removal of dust and ammonia sources help reduce respiratory irritation. Stressed animals should be handled quietly and minimally to reduce energy expenditure and oxygen demands. Keeping sick animals in familiar social groups when possible reduces stress while allowing for appropriate isolation from highly susceptible individuals.

Herd treatment protocols become necessary when BRSV outbreaks affect multiple animals simultaneously. Metaphylactic antimicrobial treatment of exposed but not yet symptomatic animals may reduce morbidity and mortality in high-risk groups such as recently weaned calves or newly arrived feedlot cattle. Anti-inflammatory protocols applied to affected groups can reduce overall disease severity. Increased monitoring frequency, sometimes requiring checks every four to six hours during acute outbreaks, enables earlier identification and treatment of new cases. Temporary cessation of cattle movements, both incoming and outgoing, prevents introduction of additional pathogens and protects naive animals from exposure.

Treatment decisions in livestock operations must balance individual animal welfare with economic realities and herd health considerations. Early identification and aggressive treatment of valuable breeding stock or high-genetic-merit animals is often economically justified despite significant treatment costs. For commercial cattle, a realistic assessment of treatment success probability guides decisions about continued treatment versus humane euthanasia or salvage slaughter where appropriate. Animals failing to respond to appropriate therapy within five to seven days generally carry a poor prognosis for full recovery. Veterinary input is essential for developing treatment protocols and making difficult decisions about individual animals, ensuring both animal welfare and economic considerations receive appropriate weight.

Recovery timeline for BRSV infection varies considerably based on disease severity, promptness of treatment, and individual animal factors. Mild cases with primarily upper respiratory involvement typically show clinical improvement within five to seven days of symptom onset, with full recovery occurring over two to three weeks. Moderate cases involving lower respiratory disease require longer recovery periods, often three to four weeks before animals return to normal appetite, activity levels, and weight gain. Severe cases complicated by extensive pneumonia, emphysema, or secondary bacterial infection may require six to eight weeks for clinical recovery, with some animals experiencing permanent respiratory compromise that affects lifetime productivity.

Post-treatment care and monitoring protocols are essential for optimal recovery outcomes. Recovered animals should remain in clean, well-ventilated housing with reduced stocking density for at least two weeks following resolution of clinical signs. Gradual return to normal feeding regimens prevents digestive upset in animals whose feed intake was reduced during illness. Continued observation for recurrence of respiratory signs enables early intervention if relapse occurs. Weight monitoring helps identify animals failing to recover appropriate growth rates despite apparent clinical improvement. These slow-gaining animals may have sustained lung damage requiring culling from breeding programs or early marketing.

Prognosis for BRSV infection depends on multiple factors including age at infection, disease severity, timing of treatment, and presence of concurrent infections. Young calves experiencing their first exposure without maternal antibody protection face the highest mortality risk, particularly when bacterial co-infection is present. Animals receiving prompt treatment at the first sign of respiratory disease demonstrate substantially better outcomes than those where intervention is delayed. Cattle previously exposed to BRSV generally experience milder disease upon reinfection due to partial immunity. Overall case fatality rates range from less than one percent in endemic herds with good management to greater than twenty percent in naive herds experiencing explosive outbreaks.

Return to production considerations are important for both dairy and beef operations following BRSV recovery. Dairy heifers recovering from severe pneumonia may experience delayed breeding and first lactation, affecting lifetime productivity and replacement program timelines. Beef calves with significant weight loss during illness require additional time on feed to reach target market weights, increasing production costs. Animals with permanent lung damage from severe BRSV infection may never achieve normal respiratory efficiency, resulting in poor performance during periods of heat stress or increased physical demands. Producers should maintain records of BRSV-affected animals and monitor their long-term productivity to inform future breeding and culling decisions.

Vaccination protocols form a cornerstone of BRSV prevention programs in cattle operations. Both modified-live and killed BRSV vaccines are commercially available, often in combination with other respiratory pathogen antigens such as IBR, BVD, and PI3. Intranasal modified-live vaccines can stimulate mucosal immunity and are appropriate for young calves even in the face of maternal antibody interference. Injectable vaccines, either modified-live or killed, are typically administered to older calves, replacement heifers, and breeding stock. Initial vaccination series generally require two doses administered three to four weeks apart, with annual boosters recommended for continued protection. Pregnant cows vaccinated in the last trimester provide enhanced passive immunity to calves through colostral antibodies.

Biosecurity measures are essential for preventing BRSV introduction and limiting spread within operations. Closed-herd management, while not always practical, provides the highest level of protection against new pathogen introduction. When cattle must be purchased, sourcing from BRSV-vaccinated herds with documented health programs reduces risk. Newly acquired animals should be quarantined for a minimum of three weeks before introduction to the main herd, allowing observation for disease development and time for vaccination to take effect. Separate equipment, clothing, and personnel for handling new arrivals versus established herd members prevents indirect transmission. Visitor access should be limited and controlled, with appropriate sanitation protocols for essential visitors.

Nutritional management contributes significantly to BRSV resistance and recovery. Cattle receiving adequate energy, protein, vitamins, and minerals maintain stronger immune function than nutritionally stressed animals. Trace mineral supplementation, particularly zinc, copper, and selenium, supports immune cell function and antibody production. Vitamin E supplementation has demonstrated benefits for respiratory health in stressed cattle. Adequate access to clean water ensures proper hydration and mucociliary function in the respiratory tract. Newly weaned calves and freshly transported cattle benefit from highly palatable starter rations that encourage feed intake during stressful transition periods.

Management practices that reduce stress and pathogen exposure provide substantial protection against BRSV and other respiratory diseases. Low-stress handling techniques, including appropriate facility design and trained personnel, minimize cortisol release that suppresses immune function. Weaning strategies such as fence-line weaning or two-stage weaning reduce stress compared to abrupt separation. Avoiding transportation during extreme weather conditions and providing adequate rest, water, and feed during and after transport supports immune competence. Maintaining appropriate stocking densities, ensuring adequate ventilation in housed cattle, and controlling dust and ammonia levels all reduce both pathogen transmission and respiratory tract irritation.

Quarantine and testing protocols provide additional layers of protection for high-value herds or operations with naive cattle populations. PCR testing of nasal swabs from incoming cattle can identify actively shedding animals before they contact the main herd. Serological testing reveals previous exposure history and helps predict susceptibility. Animals testing positive for active infection should remain isolated until repeat testing confirms viral clearance. Quarantine facilities should be physically separated from main herd housing with dedicated equipment and ideally separate airspace. Personnel should handle quarantined animals last each day and change clothing and footwear before returning to main herd areas.

Daily management and monitoring routines are essential for early BRSV detection and overall herd respiratory health. Each group of cattle should be observed at least twice daily by trained personnel who can recognize early signs of respiratory disease including subtle changes in appetite, attitude, and breathing. Morning feeding times provide excellent observation opportunities when cattle are active and approaching feed. Individual animal identification systems enable tracking of treated animals and monitoring of recovery progress. Temperature monitoring of suspect animals provides objective data for treatment decisions. Daily weather monitoring helps anticipate periods of increased respiratory disease risk and implement preventive measures.

Housing and environmental management significantly impact BRSV transmission and disease severity. Adequate ventilation providing at least four air changes per hour in enclosed housing reduces pathogen accumulation and removes noxious gases. Natural ventilation systems should be designed to provide air movement without creating drafts on cattle, particularly young calves. Stocking density should follow industry guidelines, typically providing at least thirty-five square feet per calf in group housing. Bedding should remain clean and dry, with regular removal of soiled material and addition of fresh bedding. Separate housing for different age groups prevents transmission from older cattle to susceptible young calves.

Herd health programs incorporating BRSV prevention should be developed in partnership with a licensed veterinarian familiar with the operation. Comprehensive programs include vaccination protocols tailored to the specific farm's risk factors and management system. Regular veterinary visits for herd health checks enable early problem identification and protocol adjustments. Treatment protocols specifying appropriate medications, dosages, and withdrawal times ensure consistent care when veterinary personnel are not immediately available. Necropsy of deceased animals through diagnostic laboratories provides valuable information for refining prevention and treatment approaches. Annual program reviews incorporate new diagnostic information and evaluate protocol effectiveness.

Record keeping and monitoring systems support effective BRSV management and help identify patterns requiring intervention. Individual animal health records should document all treatments, including products used, dosages, routes, and withdrawal dates. Group-level morbidity and mortality data enable tracking of disease incidence over time and comparison between management groups. Vaccination records ensure all animals receive appropriate immunizations at correct intervals. Production records including average daily gains, feed efficiency, and milk production help identify subclinical effects of respiratory disease. Electronic record systems facilitate data analysis and generation of reports for veterinary consultation and management decisions.

Economic considerations influence BRSV management decisions throughout the production cycle. Cost-benefit analysis of vaccination programs should account for direct costs of vaccines and administration labor as well as expected reductions in treatment costs, mortality, and production losses. Treatment protocols should specify decision points for discontinuing therapy in non-responsive animals to avoid excessive treatment costs with low probability of success. Insurance options for high-value cattle provide protection against catastrophic losses. Marketing timing for recovered animals requires balancing extended feeding costs against potential price premiums from additional weight gain. Facilities investments in ventilation and housing improvements should be evaluated against expected returns from reduced respiratory disease incidence.

All cattle breeds are susceptible to BRSV infection regardless of their beef or dairy purpose, though disease manifestation and management considerations may vary by breed type and production system. Holstein dairy cattle, particularly calves in intensive calf-raising facilities, frequently experience BRSV outbreaks due to high stocking densities, commingling of animals from multiple sources, and stress associated with early weaning and artificial rearing. Jersey and other dairy breeds face similar risks when raised under intensive management conditions. Beef breeds including Angus, Hereford, Charolais, and their crosses commonly encounter BRSV in feedlot settings where animals from multiple sources are combined.

Production type considerations significantly influence BRSV risk and management approaches. Dairy calves separated from dams shortly after birth miss the opportunity for extended passive antibody transfer through continued nursing, making early and adequate colostrum administration critical. Beef calves nursing dams on pasture typically receive ongoing antibody support but face increased risk at weaning when this protection is removed. Feedlot cattle entering from auction markets or order buyers face the highest risk due to stress, commingling, and potential exposure to animals from endemic herds. Seedstock operations with high-value breeding animals often implement more aggressive biosecurity and vaccination programs to protect genetic investments.

Genetic selection and testing for BRSV resistance remains an area of ongoing research rather than current practice. Unlike some hereditary conditions, no genetic test exists to identify BRSV-resistant or susceptible individuals. However, general immune competence traits are heritable to some degree, and selecting for overall animal health and longevity indirectly favors animals with robust immune systems. Some breed associations track health data that could eventually support selection for respiratory disease resistance. Crossbreeding programs often result in hybrid vigor benefits that include improved disease resistance compared to purebred animals. Producers should focus on selecting animals from herds with documented low respiratory disease incidence when possible, while recognizing that management and environmental factors typically outweigh genetic influences on BRSV disease expression.

Bovine respiratory disease complex represents the most common context for BRSV occurrence, with multiple pathogens frequently acting in concert to cause clinical pneumonia. Mannheimia haemolytica and Pasteurella multocida bacterial infections commonly follow BRSV infection, taking advantage of virus-induced damage to respiratory defenses. Histophilus somni can cause severe fibrinous pneumonia and systemic disease in BRSV-compromised animals. Mycoplasma bovis frequently complicates bovine respiratory disease, causing chronic pneumonia, arthritis, and otitis media that may persist long after the initial viral infection resolves. These bacterial co-infections often cause more severe clinical signs and higher mortality than BRSV alone.

Several viral respiratory pathogens present similar clinical signs and may occur concurrently with BRSV. Infectious bovine rhinotracheitis caused by bovine herpesvirus-1 produces upper respiratory signs including severe tracheitis and can reactivate from latent infection during stress. Bovine viral diarrhea virus causes immunosuppression that increases susceptibility to BRSV and other respiratory pathogens. Parainfluenza-3 virus produces mild respiratory disease that may be clinically indistinguishable from early BRSV infection. Bovine coronavirus occasionally causes respiratory disease in addition to its more common enteric manifestations. Diagnostic testing is often required to differentiate these viral infections and identify cases where multiple viruses are involved.

Complications and sequelae from BRSV infection can affect cattle long after the acute illness resolves. Chronic pneumonia with lung consolidation and abscessation may develop following severe bacterial co-infection. Interstitial emphysema and bullae formation can cause sudden death from pneumothorax weeks to months after apparent recovery. Permanent reduction in functional lung capacity affects heat tolerance, exercise capacity, and overall productivity. Chronic poor-doers that fail to gain weight appropriately often trace their production limitations to respiratory disease episodes earlier in life. Secondary conditions including septicemia, meningitis, and disseminated bacterial infection may develop when pathogens spread from the respiratory tract to other body systems in severely immunocompromised animals.